1. Field of the Invention
The present invention relates to an ultrasonic imaging apparatus for obtaining three-dimensional image data, a medical image-processing apparatus, and an ultrasonic image-processing method.
2. Description of the Related Art
Diagnosis whereby a medical image diagnostic apparatus obtains a three-dimensional image of a subject to be examined and the three-dimensional image is used has become popular. For example, in diagnosis using ultrasonic waves, ultrasonic imaging apparatuses described in the following (1) through (3) are used.
(1) An ultrasonic imaging apparatus that comprises a two-dimensional array probe on which ultrasonic transducers are arranged two-dimensionally (in a lattice-like pattern) and that is capable of obtaining three-dimensional image data.
(2) An ultrasonic imaging apparatus that comprises a one-dimensional array probe on which ultrasonic transducers are arranged in a predetermined direction (scanning direction) and that is capable of obtaining data three-dimensionally by mechanically swinging the one-dimensional array probe.
(3) An ultrasonic imaging apparatus that is capable of obtaining data three-dimensionally by manually moving a one-dimensional array probe.
The ultrasonic imaging apparatus of (1) and the ultrasonic imaging apparatus of (2) described above can obtain three-dimensional scanning data by transmitting ultrasonic waves three-dimensionally and receiving the reflected waves. Scanning data that has been obtained three-dimensionally is converted into voxel data by applying a three-dimensional scan conversion process.
The ultrasonic imaging apparatus of (3) can obtain two-dimensional scanning data by transmitting ultrasonic waves two-dimensionally and receiving the reflected waves. The scanning data that has been obtained two-dimensionally is converted into two-dimensional image data by applying a two-dimensional scan conversion process. Furthermore, voxel data is generated, based on a plurality of two-dimensional image data.
Then, three-dimensional image data (may be referred to as “VR image data”), MPR image data in any cross-section, or the like is generated by applying image processing such as a volume rendering process (hereinafter, may be referred to as a VR process) or an MPR process (Multiplanar Reconstruction) to the voxel data.
In a shaded volume rendering process, the vector in each voxel constituting the voxel data is found. The method in which shaded three-dimensional image data is generated, based on the vector and the voxel data is known (e.g., Japanese Unexamined Patent Publication No. 2003-61956).
A process for generating three-dimensional data will now be described referring to FIG. 1. FIG. 1 is a block diagram showing a portion of an ultrasonic imaging apparatus according to a conventional art. The case of generating three-dimensional image data (VR image data), based on scanning data that has been obtained three-dimensionally (hereinafter, may be referred to as “three-dimensional scanning data”) will be described herein.
Upon receiving three-dimensional scanning data obtained by transmitting/receiving ultrasonic waves three-dimensionally, a scan conversion processor 101 generates voxel data by applying a three-dimensional scan conversion process to the three-dimensional scanning data. Then, the scan conversion processor 101 outputs the voxel data to a vector generator 102 and a ray-tracing processor 103.
Upon receiving the voxel data from the scan conversion processor 101, the vector generator 102 finds the vector (direction) of each voxel. For example, the vector generator 102, for a certain voxel and voxels surrounding the voxel, finds the differentiation of the voxel values to find a tangent line in the voxel. Then, the vector generator 102 defines a vector perpendicular to the tangent line as the vector. Then, the vector generator 102 finds the vector of each voxel. In addition, the vector generator 102 normalizes the vector to convert the same into a unit vector. The “vector” is hereinafter described as indicating the “unit vector.”
The vector of a voxel represents the direction of the surface of a structural object included in the voxel data. Therefore, it can be seen to which direction the surface of the structural object included in the voxel data is directed by finding the vector of the voxel.
Upon receiving voxel data from the scan conversion processor 101 and furthermore receiving the vector from the vector generator 102, the ray-tracing processor 103 generates three-dimensional image data by applying a ray-tracing process to the voxel data. The ray-tracing processor 103 finds the luminance (reflection brightness) at a certain point on an object surface, based on the direction of a ray from a light source set by the ray-tracing process and the orientation of the vector calculated by the vector generator 102, thereby generating a shaded three-dimensional image data (VR image data). Specifically, the ray-tracing processor 103 finds the luminance of the object surface by finding the inner product of a vector indicating the direction of the ray from the light source and the vector.
Meanwhile, because the ultrasonic imaging apparatus generates image data by transmitting ultrasonic waves into a subject to be examined and receiving the ultrasonic waves reflected by the subject to be examined, the scanning data to be obtained depends on the state of the path through which the ultrasonic waves propagate. The path through which the ultrasonic waves propagate will now be described referring to FIG. 2. FIG. 2 is a view for illustrating the path through which the ultrasonic waves propagate. For example, as shown in FIG. 2, even in the case of transmitting the ultrasonic waves to a phantom 110 made of uniform material, a difference arises in values of obtained scanning data, depending on the presence or absence of a structural object 111 in the path of ultrasonic beams, such as in the case of ultrasonic beam 112 and ultrasonic beam 113. The structural object 111 exists in the path of the ultrasonic beam 112, but the structural object 111 does not exist in the path of the ultrasonic beam 113, so that a difference arises in obtained data between the ultrasonic beam 112 and the ultrasonic beam 113.
Further, a difference arises in values of obtained scanning data, also depending on the depth where the phantom 110 is placed. Furthermore, in the ultrasonic imaging apparatus, a speckle noise arises due to interference of ultrasonic waves effects on an image.
As described above, in the ultrasonic imaging apparatus, the obtained voxels do not always have uniform voxel values, even when a subject to be examined made of uniform material is converted into an image, and the voxel value may change abruptly at a certain location.
Even in the case of transmission of ultrasonic waves to a spatially contiguous structural object, the voxel value to be obtained changes abruptly at a certain location. As causes thereof, the following factors (1) through (3) are conceivable.
(1) The reflection coefficient of ultrasonic waves changes, depending on the difference in material of the subject to be examined.
(2) There is a difference in the state of the path through which ultrasonic waves are transmitted and received.
(3) Speckle noise is generated.
Next, effects on three-dimensional image data (VR image data) when the voxel value changes abruptly on the surface of the spatially contiguous tissues (structural object) will be described.
As described above, the vector to be used to shade a three-dimensional image can be found by differentiation of voxel values. Therefore, at the location where the voxel value changes significantly, the orientation of the vector will significantly tilt with reference to the orientation of the normal voxel in the surrounding voxels.
This tilt of the vector will now be described referring to FIG. 3. FIG. 3 is a schematic drawing for illustrating the tilt of the vector. For example, when scanning data of a certain tissue 120 are obtained, if a site 122 exists in which the value of the scanning data changes abruptly on a surface 121, the orientations of vectors 124 and 125 of the surface 121 on which the site 122 exists will significantly tilt with reference to a vector 123 of another site. For example, if there is no concavity or convexity on the surface 121, the orientation of the vector is not supposed to tilt significantly. However, because there is a location where the value of the scanning data changes abruptly, the tilt of the vector at the location will significantly tilt with reference to the orientation of the vector at another location.
As described above, although the site 122 is spatially contiguous in the tissue 120, the value of the scanning data thereof changes more abruptly compared to those of other sites, so that the orientation of the vector on the surface where the site 122 exists will significantly tilt with reference to the orientation of the vector of the surrounding sites.
When a ray-tracing process is applied to voxel data by using the vector whose orientation significantly differs from those of the surrounding vectors, a strong shadow will be locally generated on a spatially contiguous (and smooth) structural object in response to the vector with a different orientation. That is, due to the orientation of the vector significantly tilting with reference to the orientation of the vector of another location, a strong shadow will occur even at the location where the shadow is not supposed to occur. Such a shadow is an artifact that should be removed, in obstetrical diagnosis and the like.
In order to remove the shadow (artifact) described above, a smoothing process has been conventionally applied to scanning data or voxel data. This smoothing process will be described referring to FIG. 4 and FIG. 5. FIG. 4 and FIG. 5 are block diagrams for illustrating the smoothing process according to the conventional art.
As shown in FIG. 4, for example, a smoothing processor 104 applies a smoothing process, by using a predetermined smoothing filter, to three-dimensional scanning data obtained by a scan. The scan conversion processor 101 generates voxel data by applying a three-dimensional scan conversion process to the scanning data after the smoothing process. The smoothing processor 104 calculates, for example, an average of scanning data included within a predetermined range.
In addition, as shown in FIG. 5, the smoothing processor 104 applies a smoothing process, by using a predetermined smoothing filter, to the voxel data generated by the scan conversion processor 101. The vector generator 102 calculates the vector of each voxel, based on the voxel data after the smoothing process. The ray-tracing processor 103 generates three-dimensional image data, based on the vector and the voxel data after the smoothing process. The smoothing processor 104 calculates, for example, an average of voxel values within a predetermined range.
However, when the intensity of the shadow (artifact) that should be removed is strong, it is necessary to apply a smoothing process by using a strong smoothing filter. For example, it is necessary to broaden a range in which the smoothing process is performed and to strengthen the smoothing actions. Thus, a smoothing process needs to be performed by using a strong smoothing filter, so that there is a problem in that space resolution of the three-dimensional image data obtained by volume rendering is markedly reduced. In other words, when a strong smoothing filter is used to remove the shadow as an artifact, there is a problem in that the three-dimensional image becomes a blurred image.